Lean-burn gasoline engines can be more efficient and thus use less fuel and produce less carbon dioxide than corresponding engines operating under stoichiometric conditions.
One approach to treat engine emissions is to catalytically convert NO to a solid, prototypically barium nitrate, and store it in an emission control device during lean operation. The device is regenerated periodically by briefly shifting engine operation to stoichiometric or rich conditions, under which the barium nitrate becomes released NO that is then reduced. The operating temperature range for the device can be determined by the activity of the catalyst used to form the solid nitrate (defining the lower limit) and the stability of the nitrate under lean conditions (defining the upper limit). A typical range is approximately 200 to 500° C.
Although the device works well initially, its performance typically degrades over time. One reason for this is an accumulation of sulfate, derived from the combustion of fuel sulfur, which effectively competes with the nitrate for storage space. The sulfate is more stable than the nitrate, but it can be removed by an occasional exposure to rich conditions at a somewhat higher temperature than that used for normal regeneration of the trap.
The inventors herein, however, have recognized another reason for the degradation in performance of the device. Specifically, there can be a loss in activity of the catalyst used to form the solid nitrate. For example, if the catalyst is platinum supported on a high-surface-area oxide, its loss in activity can result from loss of platinum surface area due to coarsening of the supported particles of platinum. Unfortunately, known approaches for device regeneration, such as sulfur removal approaches, may not restore the platinum surface area.